The free play of an aircraft's control surfaces physically introduces decreased rigidity effects into the aeroelastic system. Such decreased rigidity effects attributable to such free play may in some cases be sufficiently large to cause limit cycle oscillations (LCO) in the surfaces that use surface rigidity to suppress flutter. The LCO can in turn reduce the aircraft fatigue life or, in certain extreme cases cause catastrophic failure. A free play test is therefore required to be conducted on an aircraft's control surfaces before an aircraft can be released for conducting a Flight Flutter Test (FFT) so that the free play is characterized and the aircraft safety is warranted.
Once the free play of a control surface is determined, according to the MIL 8870C standard (incorporated by reference hereinto), other tests are necessary in order to monitor free play evolution during the aircraft service life. These other monitoring tests of free play are known as backlash tests which are performed by the manufacturer before aircraft delivery and by the operators during aircraft service life.
In order In guarantee the aircraft safety, both the military and the FAA have published standards that define the amount of free play allowed on different aircraft control surfaces for the aircraft service life. In addition, these standards provide set points at intervals throughout the aircraft service life in which backlash must be tested.
Free play of an aircraft control surface can be tested statically or dynamically. Dynamic free play testing involves the placement of accelerometers in or on control surfaces with the surfaces thereafter being vibrated by shakers or actuators so that the free play can be monitored by a computer system. However, this type of dynamic free play testing system is generally applied to control surfaces of larger aircraft and allows for the correlation of the vibration frequency and the free play of the control surfaces which is not possible to obtain with smaller and medium sized aircraft. One such conventional dynamic free play testing system is known from U.S. Pat. No. 7,933,691 issued on Apr. 26, 2011 (the entire content of which is expressly incorporated hereinto by reference).
Because of the deficiency noted above with respect to dynamic testing, smaller and medium sized aircraft must have free play statically tested. Currently, however, static testing of the control surfaces is performed by applying a known load to the control surface and then measuring the corresponding deflection (linear measurement or angular displacement). In this regard, a typical static free play test is started a zero load and increased to some percentage of ultimate load. During testing, the moment or applied load is plotted versus displacement, i.e., to provide a L/D plot. For a control surface with no free play and a linear spring stiffness, the L/D plot is a straight line with the slope of the line being the measured spring stiffness. As free play is introduced into the system, a discontinuity in the curve occurs near the zero load range. For larger displacement values the slope increases and is more representative of the effective stiffness without the free play. As hysteresis is introduced into the system, the L/D plot forms a known type of curve.
The conventional static free play test method is timely, expensive and not very accurate. Moreover, the static test setup is relatively complex since the loading device must be fixed physically to the control surface without damaging the aircraft. Loading is typically executed with lead baskets of known mass which may in turn require certain system accommodations during each loading step thereby resulting in a very time consuming (and thus costly) process.
What has been needed therefore are improved systems and methods whereby static free play and backlash data may be obtained for control surfaces associated with small and medium sized aircraft. It is towards fulfilling such a need that the embodiments herein are directed.